The field of orthopedic surgery addresses the need to reattach tissue, particularly connective tissue such as tendons and ligaments, to bone following detachment due to injury or surgery. One approach that is commonly used is to install an implant in the bone at the reattachment site. The tissue is then tied to the implant with one or more lengths of suture. Eventually, the tissue heals by reconnecting to the bone. The implant, known as an anchor, and the sutures may be left in the body or removed.
There are several drawbacks to this procedure. Commonly-used materials for an anchor include metals, plastics, and other nonorganics that may cause adverse biochemical reactions in the body, such as bone and tissue necrosis and other damage, rejection of the implant by the body, and release of toxins into healthy tissue, bone marrow, or the blood stream through degradation of the implant. Additionally, these materials are not bioabsorbable or osteoconductive, so their permanent implantation may prevent the bone from fully healing. It would be advantageous to develop a suture anchor that has more favorable biochemical properties.
Another drawback with most existing anchors relates to securely fastening the anchor, and thereby the suture, to the bone. Some anchors are threaded like a screw and therefore are screwed into the bone, with the threads essentially cutting into the bone to secure the anchor. This can traumatize or otherwise damage the living bone when inserted and removed. Additionally, most anchors are loaded with suture before insertion. If the anchor must be twisted to seat it in the bone, pre-loaded suture will also be twisted, potentially damaging the suture. Other anchors are ribbed and are forced into a hole in the bone that is slightly smaller in diameter than the ribs. While this design may not twist the suture, it may still traumatize or otherwise damage the surrounding bone, and may also damage the anchor due to the force required to fully insert the anchor.
Push-in anchors are known to be substantially the same size as the hole into which they are inserted. Most such anchors are then secured by an adhesive. Using an adhesive, however, is potentially messy and expensive, requires an additional step, and may not be an option at all if the anchor is a temporary implant. An anchor that can be secured in place without these problems is needed.
These problems have been recently addressed using suture anchors made of bioabsorbable, osteoconductive material, including human cortical bone. These materials are absorbed by the living bone as new bone tissue develops around the implant and into its porous body. Additionally, these materials do not damage the living bone or tissue, do not release toxins, are far less likely than nonorganic materials to be rejected by the body, and allow the bone to fully heal. Typically, cortical bone material is pulverized and used as an additive in molding the anchor, but certain anchor designs made from whole cortical bone are known. For example, the Musculoskeletal Transplant Foundation produces the ALLOFIX® line of biologic suture anchors, which are machined from cortical bone of the tibia or femur of a human cadaver. It has been shown that the ALLOFIX® suture anchors are fully incorporated into the surrounding live bone and are no longer visible on x-ray films within four months of implantation. Further, it has been shown that allogenic cortical bone will naturally expand by about 3% of its size when it is inserted and contacts living bone. The mechanism of this expansion is not fully understood, but is believed to be caused largely by hydrolysis, due to the porosity of the cortical bone. The expansion of the anchor is sufficient to secure it in place, so that a push-in anchor may be used without adhesive. The expansion also encourages osteoconduction from the living bone to the anchor.
While these relatively new implants exhibit improved biochemical properties over nonorganic designs, they continue to suffer from other design drawbacks. One problem involves the use of an eyelet attached to or passing through the anchor for securing the suture to the anchor. Some anchors have eyelets attached at the top of the anchor, much like the head of a needle. These eyelets are prone to breaking off, and otherwise prevent insertion of the anchor so it is flush with the bone, because the eyelet protrudes. In other anchors, such as the ALLOFIX® anchors, the eyelet is a tunnel through the anchor body. These anchors secure the suture in place by wedging it between the anchor and the bone during insertion, a process known as interference fit. Interference overcomes the problems with protruding eyelets, but unfortunately creates a potentially more hazardous problem in “pinching” the suture. Specifically, the tensile strength of the suture may be dramatically reduced where it is pinched, and some sutures may be observably damaged when pinched due to their thickness or composition. Another drawback is the difficulty, and often impossibility, of repositioning a suture or removing a broken suture without removing the anchor. An anchor that securely retains the suture and accommodates all types of sutures without reducing the tensile strength of the suture is needed.
Through all approaches to suture anchors, a key design element is the anchor's resistance to the high tensile forces often imparted by connective tissue. The most problematic anchor susceptibilities are referred to as bending, which is the anchor's resistance to becoming concave or convex; shear, which is the anchor's resistance to lateral breakage; and pullout, which is the anchor's resistance to being withdrawn from the insertion hole. A suture anchor that addresses the drawbacks of existing anchors while maintaining acceptable resistances is desired.
Therefore, it is an object of this invention to provide an apparatus to reattach tissue that has become detached from bone. It is a further object that the device be composed of a material that is not harmful to the body. Another object of this invention is to provide an attachment device that does not damage the bone as it is inserted. A further object is to provide a suture anchor that may be used with any suture. A further object is that the suture anchor allows easy repositioning and removal of an attached suture. Another object is that the suture anchor does not reduce the tensile strength of the suture when inserted. Another object is to provide a method of attaching tissue to bone using a bioabsorbable suture anchor without reducing the tensile strength of the suture.